There is currently a great deal of concern about the health effects of nano-particles emitted unintentionally into the air. For example, the 500% increase in respiratory illness and allergies in the UK in recent years has been associated in part with particles emitted by diesel engines and other combustion processes. Whilst the main focus has been on diesel emissions, attention is turning to other potential sources such as power generation using fossil fuels, incineration, nuclear power generation and aircraft emissions. All heavy industries involving processes emitting fumes have potential problems with the emission of nano-particles. Such processes include smelting, firing, glass manufacture, welding, soldering, nuclear power generation and incineration. There is also concern amongst consumer companies that enzymes in washing powders, powder coatings and fibres used in disposable nappies and other products could cause problems. In addition, the US EPA is becoming increasingly concerned about gasoline engine emissions.
Nano-particles are known to produce toxic effects. For example, they can cross the blood-brain barrier in humans and gold nano-particles can move across the placenta from mother to foetus. Early studies with PTFE (polytetrafluoroethylene) particles around 20 nm in diameter showed that airborne concentrations of a supposedly inert insoluble material lower than 50 μg/m3 could be fatal to rats. Moreover, nano-tubes produce a more toxic response in rats than quartz dust.
In addition to concerns from a health perspective, the elimination or control of airborne nano-particles is important in maintaining standards in the many thousands of clean rooms in the micro-electronics, pharmaceutical, medical, laser, and fibre optics industries.
Small particles can be classified as shown in Table 1 below.
TABLE 1TermAerodynamic Particle Size RangeDustD > 10 μmCoarse particles2.5 μm < D < 10 μmFine particles0.1 μm < D < 2.5 μmNano-particles or ultrafine particles1 nm < D < 0.1 μm
The term “nano-particles” is used to refer to particles having an aerodynamic particle size in the range from 1 nm to 0.1 μm (100 nm).
For spherical particles, the aerodynamic particle size is the diameter of the particle. Real particles in the air often have complicated shapes. For non-spherical particles, the term “diameter” is not strictly applicable. For example, a flake or a fibre has different dimensions in different directions. Particles of identical shape can be composed of different chemical substances and have different densities. The differences in shape and density cause considerable confusion in defining particle size.
The terms “aerodynamic particle size” or “aerodynamic diameter” are therefore used in order to provide a single parameter for describing real non-spherical particles having arbitrary shapes and densities. As used herein, the term “aerodynamic diameter” is the diameter of a spherical particle having a density of 1 g/cm3 that has the same inertial property (terminal settling velocity) in the air (at standard temperature and pressure) as the particle of interest. Inertial sampling instruments such as cascade impactors enable the aerodynamic diameter to be determined. The term “aerodynamic diameter” is convenient for all particles including clusters and aggregates of any forms and density. However, it is not a true geometric size because non-spherical particles usually have a lower terminal settling velocity than spherical particles. Another convenient equivalent diameter is the diffusion diameter or thermodynamic diameter which is defined as a sphere of 1 g/cm3 density that has the same diffusivity as a particle of interest.
The investigation and monitoring of nano-particles in the atmosphere has been hampered by a shortage of instruments which can measure in the nano-particle range but which are sufficiently inexpensive, robust and convenient to be used on a widespread basis.
Some instruments for measuring nano-particles are known which make use of laser optics to detect and measure particles. However, because optical measurements cannot readily be used to detect particles in the nano-particle size range, techniques have been developed for “growing” particles to make them larger and therefore detectable and this technique forms the basis for Condensation Particle Counters. Condensation Particle Counters (CPCs) work by passing a sample of airborne particles through a chamber containing a vapourised liquid and then through a condenser where the vapourised liquid is condensed onto the airborne particles to form droplets of a size that can be measured. One example of such an instrument is disclosed in WO 02/029382 (Ahn et al). The CPC disclosed in WO 02/029382 comprises a cylindrical evaporation chamber which is lined with a porous absorbent support formed from a material such as nonwoven fabric. At one end of the chamber, the porous absorbent support is in contact with a reservoir of a volatile liquid such as isobutanol so that the liquid can travel along and soak the support by capillary action. The exterior surface of the evaporation chamber is surrounded by a heating element that heats the chamber causing isobutanol to evaporate from the support thereby to create a vapour-filled chamber. Air samples suspected of containing airborne particles are introduced into the chamber at the reservoir end and drawn through the chamber into a condenser where the condensation of the isobutanol vapour onto the airborne particles takes place to form droplets that can be measured using an optical particle counter.
An example of a commercially available CPC making use of the principles disclosed in WO 02/029382 is the Model 3025A Ultrafine Condensation Particle Counter available from TSI Incorporated, Shoreview, Minn., U.S.A.
Another known apparatus is the handheld CPC 3007 from TSI (www.tsi.com), and the operation of this is described in more detail below in relation to FIG. 1.
Existing Condensation Particle Counters suffer from a number of disadvantages. For example, they tend to require a high power consumption in order to heat the working fluid and have a long (10 to 20 minutes) warming up time before they can be used. These disadvantages arise at least in part because the evaporation chamber is heated by means of an external heating element and therefore the entire casing surrounding the chamber must heated before the instrument reaches the operating temperature. Furthermore, with known CPCs, there is a relatively high consumption of the working fluid (e.g. isobutanol) with the result that the working fluid must be topped up on a frequent basis, often before each use. Even in the case of the TSI US 3007 handheld condensation particle counter, the working fluid cartridge with the working fluid must be replaced on a regular basis. A further disadvantage of known CPCs is the unpleasant smell of the working fluids used (e.g. iso-butanol) and the relatively high costs.
At present, therefore, there remains a need for a Condensation Particle Counter that can be used for long periods without topping up the working fluid, which has a greatly reduced warm-up time and which lends itself to miniaturisation.